The present invention relates to a PTC resistor device, which stands for a Positive Temperature Coefficient thermistor resistor. A PTC device is used as a current control device in a motor starter, an oil evaporation device in an oil furnace, a thermo-bottle, and/or a heater for a mosquito stick.
A PTC resistor has the characteristics that the resistance is high at high temperature, therefore, there is no danger of overheating of a heater, because power consumption is reduced automatically at high temperature. A PTC element is made of mainly barium titanate.
There have been some inventions relating to a PTC device. The U.S. Pat. No. 4,232,214 which is filed by the present assignee is one of them.
Conventionally, a PTC resistor has the structure comprising a flat PTC plate, silver electrode layers attached on both the surfaces of said PTC plate, and, a pair of electrodes coupled electrically with said silver layers. However, that conventional structure has the disadvantage of a silver migration effect, in which a silver molecule of a silver layer moves from the silver layer to the PTC plate along the outer surface of the plate when a voltage is applied between the electrodes of the PTC resistor, and the electrodes are finally short-circuited. That silver migration effect is considerable at high temperature condition. The silver migration effect could be overcome if the electode layers were replaced by a gold layer, platinum layer, or palladium layer, however, the replacement by those materials would cause the increase of the production cost of a PTC resistor, and therefore, these replacements are not preferable.
Another prior system for overcomig the silver migration effect is to provide an anti-silver migration barrier with another metal on a silver layer. However, that technique has the disadvantage that the production cost of a PTC resistor is also increased, and/or the barrier layer is not stable at high temperature.
Accordingly, we have developed a new structure of a PTC which is expected to be free from silver migration. The structure of that PTC device is shown in FIGS. 1 through 3, in which the PTC heater comprises the PTC plate 1, a pair of first conductive layers 2 which are made of a metal that is not silver and contact with the PTC plate with ohmic contact, and a second pair of conductive layers 3 which are made mainly of silver and are attached on the first layers so that a gap G is left at the peripheral circle of the first layers 2. That structure is free from silver migration, since the first layers which contact directly with the PTC plate is made of another metal, not silver, and the PTC plate holds the positive temperature coefficient characteristics since the first layers provide the ohmic contact with the PTC plate. Further, since the first layers 2 are provided on the whole surfaces of the PTC plate, the current density in the PTC plate can be uniform, and therefore, the heat generation in the PTC plate is also uniform. Those first layers 2 are provided through electroless plating process, ion plating process, sputtering process, or screen printing process.
Further, since second layers 3 are provided on the first layers 2, the conductivity of the combined electrodes are low in spite of the high conductivity of the second layers which include silver. It should be noted that the second layers which are made of silver do not contact directly with a PTC plate, since a gap G is provided at the peripheral of the layers, and therefore, the silver migration by the second layers is prevented. Preferably, the width of said gap space G is approximately 0.1.about.4 mm.
However, we found that the structure of FIGS. 1 and 2 has the disadvantage as described in accordance with FIG. 3, in which the numerals 4 and 5 are terminal electrodes for coupling the PTC heater with an external circuit, and said electrodes 4 and 5 contact with the second layers at almost all the area of the second layers. In the structure of FIG. 3, when the electrodes 4 and 5 are curved or deformed by high temperature due to the heating of the PTC resistor, the electrodes 4 and 5 might contact with the first layers only at the portion of the gap G. It should be noted that the resistance of the first layers is high since they are made of a metal that is not silver (some examples of the first layers are nickle, brass, or aluminum, and therefore, the contact resistance at the gap G between the electrodes 4 and 5, and the first layers is rather high, and therefore, the portion of the gap G is partially excessively heated. Then, the PTC heater itself is broken by that partial heat loss. That disadvantage comes from the partial contact of the electrodes 4 and 5 with the first layers at the gap G.